It is desirable to provide high precision printed circuit boards in many situations, including both printed circuit boards that are double-sided and boards which have multiple layers. The fact that a printed circuit board has high precision circuit paths enables that board to also have a high density of those circuit paths. Such circuit path high density is normally the ultimate goal in manufacturing multiple layered printed circuit boards.
One example of a printed circuit board having high density is U.S. Pat. No. 4,306,925 by Lebow. Lebow discloses a method to make high density printed circuit boards including a step where a metallic conductive circuit pattern is formed using photolithography onto a smooth substrate. A patent by Robertson, U.S. Pat. No. 4,600,663, discloses a method for forming a microstrip line having precise width and precise edge definition onto a substrate. Robertson would also lend itself to manufacturing devices having high density.
Another method of achieving high density in printed circuit boards is to make a printed circuit board which has multiple circuit path layers. U.S. Pat. No. 4,631,100 by Pellegrino discloses a method for mass producing printed circuit boards, including those having multiple layers. Pellegrino discloses a printed circuit board blank which has a predetermined pattern of pads and interconnecting conductive pathways. To fabricate a finished circuit board of any desired circuit configuration, the printed circuit board blank is coated with a photoresist and exposed so that upon development of the photoresist, and upon being etched in accordance with the developed pattern, the interconnecting conductive pathways between pads is selectively etched away so that only those interconnects for the desired circuit pattern remain. Pellegrino does not, however, provide information as to how to achieve a higher yield of multi-layer printed circuit boards. When manufacturing printed circuit boards having six, or a maximum of eight, layers, printed circuit board manufacturers are fortunate to achieve first pass production yields of 10-20%. It is obvious that an invention that would dramatically improve the yield of multi-layer printed circuit boards would be quite useful in industry, and would dramatically cut down on the amount of wasted materials used.
Multi-layer printed circuit boards which are manufactured using current process technologies undergo the following process steps (See FIG. 2):
(1) Material ordering and receiving 40. PA1 (2) Material storage 42. PA1 (3) Material selection 44. PA1 (4) Processing of inner layers 46. PA1 (5) Lamination 48. PA1 (6) Processing of outer layers 50. PA1 (7) Final quality control 52. PA1 (1) Material ordering and receiving 80. PA1 (2) Material storage 82. PA1 (3) Material selection 84. PA1 (4) Processing of double-sided board 86. PA1 (5) Final quality control 88. PA1 (1) Maintaining a distance between copper layers in the board with no more than a +/- 0.0015" (0.0381 mm) variance between board layers. PA1 (2) Maintaining copper trace stability to +/- 0.0005" (0.0127 mm) of a specified line width. PA1 (3) Maintaining an outer dimensional stability of +/- 0.010" (0.254 mm) over a 17" (431.8 mm) span. PA1 (4) Maintaining a plating tolerance of +/- 0.000001" (0.0000254 mm) from a specified thickness on the sub-tracks. PA1 (5) Computing a shrink correction factor for each lot of substrates so that each of the completed boards all meet the final dimensional tolerances. PA1 (6) Maintaining a dielectric constant of 3.2 or lower in final constructed form for the final laminated board. PA1 (7) Creation of printed wiring boards which are dimensionally stable to 150.degree. C. and structurally stable to 245.degree. C.
The use of this seven-step method is well known in the art, and produces about a 10% yield in the first pass of manufacturing. Multi-layer printed circuit boards of up to eight layers are made using current process technologies to an accuracy of 2 mils (0.0508 mm) for line thickness and line spacing. This 2 mil accuracy is the best accuracy that is being achieved using the current process technologies. Multi-layer printed circuit boards which have greater than eight layers can be made using current process technologies only to a much coarser accuracy, such as 8 mils for line thickness and line spacing.
The reason 2 mil accuracy is the best that it is currently being achieved is mostly due to "drift." During the lamination process step of manufacturing a multi-layer printed circuit board, the fiberglass which makes up the major portion of the substrate of each of the layers has a tendency to drift in the direction of its grain. Because of this drift, the maximum number of multiple layers currently being produced with a 2 mil accuracy is eight layers. The greater the number of layers, the more difficult it is to control the registration points of each of the layers with respect to the other layers. Since the registration points are so difficult to line up, the effect is to limit the inaccuracy which would occur when attempting to use smaller line sizes and line spacings than 2 mils.
Double-layer printed circuit boards which were manufactured using current process technologies undergo similar process steps as compared to the processing of multi-layer printed circuit boards, as follows (See FIG. 4):
Using current process technologies, double-sided printed circuit boards also suffer somewhat to drift. As a result, such printed circuit boards are currently being produced with a 2 mil accuracy, similar to that being produced in multi-layer printed circuit boards.
With the limitations of the current process technologies, it would be impossible to achieve the fine accuracy required to manufacture multi-layer printed circuit boards that would be precise enough to be formed into coaxial cable equivalents or tuned wave guide equivalents that are built right into the printed circuit boards themselves. In order to make a high performance system having the above capabilities, very tight controlling impedances must be maintained. To achieve these very tight controlled impedances, the necessary method must include the following capabilities:
A multi-layer printed circuit board that is manufactured to the above tolerances can be used as a backplane in a system which requires the equivalent of high density coaxial cable connections between the various elements of the system. In other words, such a printed circuit board would be dimensionally and electrically stable enough to create coaxial cable equivalents or tuned wave guide equivalents, and would also have a high enough density to allow placement of at least 100 tuned wave guides per inch. Such multi-layer printed circuit boards are not capable of being produced under the current process technologies.